The present invention relates to multilayer coextruded light-reflecting films which have a narrow reflection band because of light interference. When the reflection band occurs within the range of visible wavelength, the film is iridescent. Similarly, when the reflection band falls outside the range of visible wavelength, the film is either ultraviolet or infrared reflecting.
The multilayer films and methods by which they can be produced are known in the art. In this connection, the reader's attention is directed to the following United States patents which are hereby incorporated by reference: U.S. Pat. Nos. 3,328,003; 3,442,755; 3,448,183; 3,479,425; 3,480,502; 3,487,505; 3,511,903; 3,549,405; 3,555,128; 3,557,265; 3,565,985; 3,576,707; 3,647,612; 3,711,176; 3,759,647; 3,773,882; and 3,801,429.
The multilayer films are composed of a plurality of generally parallel layers of transparent thermoplastic resinous material in which the contiguous adjacent layers are of diverse resinous material whose index of refraction differs by at least about 0.03. The films contain at least 10 layers and more usually at least 50 layers and, preferably, at least about 90 layers.
The individual layers of the film are very thin, usually in the range of about 30 to 500 nm, preferably about 50-400 nm, which causes constructive interference in light waves reflected from the many interfaces. Depending on the layer thickness and the refractive index of the polymers, one dominant wavelength band is reflected and the remaining light is transmitted through the film. The reflected wavelength is proportional to the sum of the optical thicknesses of a pair of layers. The reflected wavelength can be calculated by the formula EQU .lambda..sub.M =M/2 (n.sub.1 t.sub.1 +n.sub.2 t.sub.2)
In this formula, .lambda. is the reflected wavelength, M is the order of reflection, t is the layer thickness, n is the refractive index, and 1 and 2 indicate the polymer of the first layer and the polymer of the second layer, respectively. The quantity nt is the optical thickness of a layer. For first order reflections, i.e. when M is 1, visible light is reflected when the sum of optical thicknesses falls between about 200 and 350 nm. When the sum is lower than about 200, the reflection is in the ultraviolet region of spectrum and when the sum is greater than about 350 nm, the reflection is in the infrared region.
The quantity of the reflected light (reflectance) and the color intensity depend on the difference between the two refractive indexes, on the ratio of optical thicknesses of the layers, on the number of layers and on the uniformity of the thicknesses. If the refractive indexes are the same, there is no reflection at all from the interfaces between the layers. In the multilayer films, the refractive indexes of contiguous adjacent layers differ by at least 0.03 and preferably by at least 0.06 or more. For first order reflections, reflectance is highest when the optical thicknesses of the layers are equal although suitably high reflectances can be achieved when the ratio of the two optical thicknesses falls between 5:95 and 95:5. Distinctly colored reflections are obtained with as few as 10 layers, however, for maximum color intensity it is desired to have between 50 and 1000 or even more layers. High color intensity is associated with a reflection band which is relatively narrow and which has high reflectance at its peak. It should be recognized that although the term "color intensity" has been used here for convenience, the same considerations apply to the invisible reflection in the ultraviolet and infrared ranges.
The chill roll multilayer cast films are made using a conventional single manifold flat film die in combination with a feedblock which collects the melts from each of two or more extruders and arranges them into the desired layer pattern. Feedblocks are described in the aforementioned U.S. Pat. Nos. 3,565,985 and 3,773,882. The feedblocks can be used to form alternating layers of either two components (i.e. ABAB . . . ) or three components (ABCABCA . . . ) or more. The very narrow multilayer stream flows through a single manifold flat film die where the layers are simultaneously spread to the width of the die and thinned to the final die exit thickness. The number of layers and their thickness distribution can be changed by inserting a different feedport module.
In order to achieve good control of layer uniformity, the outside surface or skin layers should be thicker than the intermediate optical layers. If the skin layers are too thin, some of the interfaces of the optical layers can be subjected to high shear stress resulting in interfacial instability which disrupts the regular laminar flow of the optical layers. It is therefore necessary for the surface skins to be thick enough to prevent interfacial instability at the layer interfaces. It has been found that the two skin layers should together have a thickness which is at least 5% of the thickness of the multilayer core and can be up to several times the thickness of the multilayer core. It is normally not desirable for the total skin thickness to be more than three times that of the multilayer core for economic reasons.
A wide variety of transparent, plastic resinous material can be used to form the layers of the multilayer light-reflecting film. Some usable polymers and their refractive index are set forth in the following Table.
TABLE 1 ______________________________________ Refractive Polymer name: index ______________________________________ Polytetrafluoroethylene 1.35 FEP (fluorinated ethylene-propylene copolymer) 1.34 Polyvinylidenefluoride 1.42 Polychlorotrifluoroethylene 1.42 Polybutyl acrylate 1.46 Polyvinyl acetate 1.47 Ethyl cellulose 1.47 Polyformaldehyde 1.48 Polyisobutyl methacrylate 1.48 Polybutyl methacrylate 1.48 Polymethyl acrylate 1.48 Polypropyl methacrylate 1.48 Polyethyl methacrylate 1.48 Polymethyl methacrylate 1.49 Cellulose acetate 1.49 Cellulose propionate 1.49 Cellulose acetate-butyrate 1.49 Cellulose nitrate 1.49 Polyvinyl butyral 1.49 Polypropylene 1.49 Low density polyethylene (branched) 1.51 Polyisobutylene 1.51 Natural rubber 1.52 Perbunan 1.52 Polybutadiene 1.52 Nylon (condensation copolymer of hexa- methylene-diamine and adipic acid) 1.53 Polyvinyl chloroacetate 1.54 Polyvinylchloride 1.54 Polyethylene (high density linear) 1.54 A copolymer of 67 parts by weight methyl methacrylate and 33 parts by weight styrene 1.54 A copolymer of 85 parts by weight vinyl chloride and 15 parts by weight vinyl- idene chlorride 1.55 Poly-.alpha.-methylstyrene 1.56 A copolymer of 60 parts by weight styrene and 40 parts by weight butadiene 1.56 Neoprene 1.56 A copolymer of 70 parts by weight styrene and 30 parts by weight acrylonitrile 1.57 Polycarbonate resin 1.59 Polystyrene 1.60 A copolymer of 85 parts by weight vinyl- idene chloride and 15 parts by weight vinyl chloride 1.61 Polydichlorostyrene 1.62 ______________________________________
The use of polyolefins as one of the diverse resinous materials in the multilayer light-reflecting film has been found advantageous. These polymers, such as low density polyethylene, high density polyethylene, propyleneethylene copolymer, polypropylene homopolymer, and the like, have refractive indexes which are approximately midway between low index polymers such as the fluorine-containing polymers and high index polymers such as polystyrene, polycarbonate, vinylidene chloride copolymer and the like. Therefore, the necessary 0.03 refractive index difference can be established in either direction. Additionally, when used with the relatively brittle polystyrene, the polyolefin imparts desirable flexibility and toughness which makes it possible to wind the film on rolls with less difficulty and to feed it into continuous film handling and converting equipment.
Unfortunately, there are disadvantages inherent in polyolefin containing film. The adhesion forces between the layers of polyolefin and the adjacent polymer are rather small. As a result, under some conditions of use such as, for example, after laminating the multilayer film to a substrate such as paper, paperboard, plastic film or sheet or the like, the multilayer film shows some tendency to internal delamination. Additionally, in forming the film, the skin layer and multilayer core fail to uniformly spread from the original very compact stream, typically about 2.5 cm in cross-section, to the full width of the die which can be as wide as 120 cm or greater. As a result, the skin layers are generally thicker at the edges of the wide film than at the center of the film even though the total thickness of the cast film is kept uniform from edge to center in order to facilitate winding and handling. It will be appreciated that this results in the multilayer core being compressed at the edges of the film with respect to the center and since the number of layers remains constant, the individual layers are thinner at the edges of the film.
An iridescent film which suffers from this defect may be, for example, red-reflecting in the center and blue- or violet-reflecting at the edges because of the differences in optical layer thicknesses. If the film is to be used for decorative purposes, it is most often desired to have the same color reflecting characteristics throughout. If the film is to be used for applications requiring ultraviolet or infrared reflection, it is important that the entire area have the same reflection characteristics.
Accordingly, it is the object of this invention to provide a solution to the two problems just described with respect to polyolefin-containing film, i.e. the inadequate adhesion between layers and the lack of uniform thickness from center to edge of the film. This and other objects of the invention will become apparent to those skilled in this art from the following detailed description.